Search results for " edge"
showing 10 items of 226 documents
Titanium dioxide surface stoichiometry and ordering studied by resonant photoemission spectroscopy
2005
Abstract The electronic structure of titanium dioxide surfaces having undergone different preparations leading to different stoichiometries and crystallinities has been studied using resonant photoemission spectroscopy. Valence band photoemission spectra through the Ti 3p–3d/4s absorption edge between 45 and 55 eV were measured and allowed a characterization of defects present at the surface as well as of the quality of the surface organization. Indeed, from the comparison of the resonance results obtained for each kind of surface with the LEED patterns on the one hand and the corresponding Ti 2p core level lines on the other hand, it was evidenced that the high binding energy part of the v…
Current status of AlInN layers lattice-matched to GaN for photonics and electronics
2007
We report on the current properties of Al1-x InxN (x approximate to 0.18) layers lattice- matched ( LM) to GaN and their specific use to realize nearly strain- free structures for photonic and electronic applications. Following a literature survey of the general properties of AlInN layers, structural and optical properties of thin state- of- the- art AlInN layers LM to GaN are described showing that despite improved structural properties these layers are still characterized by a typical background donor concentration of ( 1 - 5) x 10(18) cm(-3) and a large Stokes shift (similar to 800 meV) between luminescence and absorption edge. The use of these AlInN layers LM to GaN is then exemplified …
Luminescence of biexcitons in silver halide crystals
1998
Abstract As it is known from Urbach rule, free excitons defining the fundamental absorption edge in AgBr 1 − x Cl x (0 ⩽ x ⩽ 1). The process of primary Frenkel defect creation which involves free exciton states even in AgCl and biexciton creation in AgCl are discussed. The fast luminescence band at ≈3.0 eV observed in AgCl under high-density pulsed electron beam and laser excitation has been studied. It is shown that its intensity is a non-linear function of the excitation density. We suggest that the fast luminescence comes from biexcitons; binding energies of such biexcitons in AgBr 1 − x Cl x (0 ⩽ x ⩽ 1) are estimated.
QUADRUPOLAR CHARACTER OF THE Ti K-EDGE PREPEAKS IN TiO2 BY RESONANT AUGER
2002
Resonant spectroscopies offer a new opportunity to get more insight into excited electronic states by studying line shapes and intensities of decay processes. For photon excitations in the pre-K-edge region, the Ti KL 2,3 L 2,3 Auger spectra in TiO 2 show additional peaks when an electron is promoted in localized d-like states via a quadrupolar transition. This resonant process is used to unravel the respective contributions of quadrupolar and dipolar transitions to the absorption edge prepeaks.
Non-Markovian dynamics from band edge effects and static disorder
2017
It was recently shown [S. Lorenzo et al., Sci. Rep. 7, 42729 (2017)] that the presence of static disorder in a bosonic bath - whose normal modes thus become all Anderson-localised - leads to non-Markovianity in the emission of an atom weakly coupled to it (a process which in absence of disorder is fully Markovian). Here, we extend the above analysis beyond the weak-coupling regime for a finite-band bath so as to account for band edge effects. We study the interplay of these with static disorder in the emergence of non-Markovian behaviour in terms of a suitable non-Markovianity measure.
Excitonic absorption and Urbach's tail in bismuth sulfide single crystals
1988
The absorption coefficient of bismuth sulfide single crystals has been measured through more than four orders of magnitude and in the range of energies from 1.25 to 1.70 eV. A detailed study as a function of temperature has been carried out from 29 to 300 K. An Urbach tail for low values of absorption has been found. This tail and its temperature evolution fit the expression for ionic materials. An excitonic region appears at low temperature and the shape of the exciton peak is Gaussian, which corresponds to a strong exciton-phonon coupling. The exciton binding energy is estimated (28±3 meV) and then the energy gap at 29 K is obtained (Eg=1.523±0.003 eV). The fundamental electronic transiti…
Ab-Initio Studies of Electronic and Spectroscopic Properties of MgO, ZnO and CdO
2008
We present ab-initio calculations of excited-state properties within single-particle and two-particle approaches in comparison with corresponding experimental results. For the theoretical treatment of the electronic structure, we compute eigenvalues and eigenfunctions by using a spatially nonlocal exchange-correlation potential. From this starting point, quasiparticle energies within the fully frequency-dependent G(0)W(0) approximation are obtained. By solving the Bethe-Salpeter equation, we evaluate optical properties, including the electron-hole attraction and the local-field effects. The results are compared with experimental spectra from soft X-ray emission, as well as from X-ray photoe…
Disappearance of Hard X-Ray Emission in the Last BeppoSAX Observation of the Z Source GX 349+2
2003
We report on the results from two BeppoSAX observations of the Z source GX 349+2 performed in February 2001 and covering the broad energy range 0.12-200 keV. The light curve obtained from these observations shows a large flaring activity, the count rate varying from ~130 to ~260 counts/s, indicating that the source was in the flaring branch during these observations. The average spectrum is well described by a soft blackbody and a Comptonized component. To well fit the energy spectrum three gaussian lines are needed at 1.2 keV, 2.6 keV, and 6.7 keV with corresponding equivalent widths of 13 eV, 10 eV, and 39 eV, probably associated to L-shell emission of Fe XXIV, Ly-alpha S XVI, and Fe XXV,…
A Hard Tail in the X‐Ray Broadband Spectrum of Circinus X‐1 at the Periastron: A Peculiar Z Source
2001
We report on the spectral analysis of the peculiar source Cir X-1 observed by the BeppoSAX satellite when the X-ray source was near the periastron. A flare lasting ~6 × 103 s is present at the beginning of the observation. The luminosity during the persistent emission is 1 × 1038 ergs s-1, while during the flare it is 2 × 1038 ergs s-1. We produced broadband (0.1-100 keV) energy spectra during the flare and the persistent emission. At low energies the continuum is well fitted by a model consisting of Comptonization of soft photons, with a temperature of ~0.4 keV, by electrons at a temperature of ~1 keV. After the flare, a power-law component with photon index ~3 is dominant at energies high…
Spectral Evolution of Circinus X-1 along Its Orbit
2001
We report on the spectral analysis of Circinus X-1 observed by the ASCA satellite in March 1998 along one orbital period. The luminosity of the source (in the 0.1-100 keV band) ranges from $2.5 \times 10^{38}$ erg s$^{-1}$ at the periastron (orbital phase 0.01) to $1.5 \times 10^{38}$ erg s$^{-1}$ at orbital phase 0.3. From the spectral analysis and the lightcurve we argue that Cir X-1 shows three states along the orbital evolution. The first state is at the orbital phase interval 0.97-0.3: the luminosity becames super-Eddington and a strong flaring activity is present. In this state a shock could form in the inner region of the system due to the super-Eddington accretion rate, producing an…